The Digestive System

1
Chapter 23

• The Digestive System

2
Digestive System Overview

• The alimentary canal or gastrointestinal (GI)
  tract digests and absorbs food
• Alimentary canal mouth, pharynx, esophagus,
  stomach, small intestine, and large intestine
• Accessory digestive organs teeth, tongue,
  gallbladder, salivary glands, liver, and pancreas

3
Figure 23.1
4
Digestive Process

• The GI tract is a disassembly line
• Nutrients become more available to the body in
  each step
• There are six essential activities
• Ingestion, propulsion, and mechanical digestion
• Chemical digestion, absorption, and defecation

5
Figure 23.2
6
Gastrointestinal Tract Activities

• Ingestion taking food into the digestive tract
• Propulsion swallowing and peristalsis
• Peristalsis waves of contraction and relaxation
  of muscles in the organ walls
• Mechanical digestion chewing, mixing, and
  churning food

7
Peristalsis and Segmentation
Figure 23.3
8
Gastrointestinal Tract Activities

• Chemical digestion catabolic breakdown of food
• Absorption movement of nutrients from the GI
  tract to the blood or lymph
• Defecation elimination of indigestible solid
  wastes

9
GI Tract

• External environment for the digestive process
• Regulation of digestion involves
• Mechanical and chemical stimuli stretch
  receptors, osmolarity, and presence of substrate
  in the lumen
• Extrinsic control by CNS centers
• Intrinsic control by local centers

10
Receptors of the GI Tract

• Mechano- and chemoreceptors respond to
• Stretch, osmolarity, and pH
• Presence of substrate, and end products of
  digestion
• They initiate reflexes that
• Activate or inhibit digestive glands
• Mix lumen contents and move them along

11
Nervous Control of the GI Tract

• Intrinsic controls
• Nerve plexuses near the GI tract initiate short
  reflexes
• Short reflexes are mediated by local enteric
  plexuses (gut brain)
• Extrinsic controls
• Long reflexes arising within or outside the GI
  tract
• CNS centers and extrinsic autonomic nerves

12
Nervous Control of the GI Tract
Figure 23.4
13
Peritoneum and Peritoneal Cavity

• Peritoneum serous membrane of the abdominal
  cavity
• Visceral covers external surface of most
  digestive organs
• Parietal lines the body wall
• Peritoneal cavity
• Lubricates digestive organs
• Allows them to slide across one another

14
Peritoneum and Peritoneal Cavity
Figure 23.5a
15
Peritoneum and Peritoneal Cavity

• Mesentery double layer of peritoneum that
  provides
• Vascular and nerve supplies to the viscera
• Hold digestive organs in place and store fat
• Retroperitoneal organs organs outside the
  peritoneum
• Peritoneal organs (intraperitoneal) organs
  surrounded by peritoneum

16
Peritoneum and Peritoneal Cavity
Figure 23.5b
17
Blood Supply Splanchnic Circulation

• Arteries and the organs they serve include
• The hepatic, splenic, and left gastric spleen,
  liver, and stomach
• Inferior and superior mesenteric small and large
  intestines

18
Blood Supply Splanchnic Circulation

• Hepatic portal circulation
• Collects nutrient-rich venous blood from the
  digestive viscera
• Delivers this blood to the liver for metabolic
  processing and storage

19
Histology of the Alimentary Canal

• From esophagus to the anal canal the walls of the
  GI tract have the same four tunics
• From the lumen outward they are the mucosa,
  submucosa, muscularis externa, and serosa
• Each tunic has a predominant tissue type and a
  specific digestive function

20
Histology of the Alimentary Canal
Figure 23.6
21
Mucosa

• Moist epithelial layer that lines the lumen of
  the alimentary canal
• Three major functions
• Secretion of mucus
• Absorption of end products of digestion
• Protection against infectious disease
• Consists of three layers a lining epithelium,
  lamina propria, and muscularis mucosae

22
Mucosa Epithelial Lining

• Simple columnar epithelium and mucus-secreting
  goblet cells
• Mucus secretions
• Protect digestive organs from digesting
  themselves
• Ease food along the tract
• Stomach and small intestine mucosa contain
• Enzyme-secreting cells
• Hormone-secreting cells (making them endocrine
  and digestive organs)

23
Mucosa Lamina Propria and Muscularis Mucosae

• Lamina Propria
• Loose areolar and reticular connective tissue
• Nourishes the epithelium and absorbs nutrients
• Contains lymph nodes (part of MALT) important in
  defense against bacteria
• Muscularis mucosae smooth muscle cells that
  produce local movements of mucosa

24
Mucosa Other Sublayers

• Submucosa dense connective tissue containing
  elastic fibers, blood and lymphatic vessels,
  lymph nodes, and nerves
• Muscularis externa responsible for segmentation
  and peristalsis
• Serosa the protective visceral peritoneum
• Replaced by the fibrous adventitia in the
  esophagus
• Retroperitoneal organs have both an adventitia
  and serosa

25
Enteric Nervous System

• Composed of two major intrinsic nerve plexuses
• Submucosal nerve plexus regulates glands and
  smooth muscle in the mucosa
• Myenteric nerve plexus Major nerve supply that
  controls GI tract mobility
• Segmentation and peristalsis are largely
  automatic involving local reflex arcs
• Linked to the CNS via long autonomic reflex arc

26
Mouth

• Oral or buccal cavity
• Is bounded by lips, cheeks, palate, and tongue
• Has the oral orifice as its anterior opening
• Is continuous with the oropharynx posteriorly

27
Mouth

• To withstand abrasions
• The mouth is lined with stratified squamous
  epithelium
• The gums, hard palate, and dorsum of the tongue
  are slightly keratinized

28
Anatomy of the Oral Cavity Mouth
Figure 23.7a
29
Lips and Cheeks

• Have a core of skeletal muscles
• Lips orbicularis oris
• Cheeks buccinators
• Vestibule bounded by the lips and cheeks
  externally, and teeth and gums internally
• Oral cavity proper area that lies within the
  teeth and gums
• Labial frenulum median fold that joins the
  internal aspect of each lip to the gum

30
Oral Cavity and Pharynx Anterior View
Figure 23.7b
31
Palate

• Hard palate underlain by palatine bones and
  palatine processes of the maxillae
• Assists the tongue in chewing
• Slightly corrugated on either side of the raphe
  (midline ridge)

32
Palate

• Soft palate mobile fold formed mostly of
  skeletal muscle
• Closes off the nasopharynx during swallowing
• Uvula projects downward from its free edge
• Palatoglossal and palatopharyngeal arches form
  the borders of the fauces

33
Tongue

• Occupies the floor of the mouth and fills the
  oral cavity when mouth is closed
• Functions include
• Gripping and repositioning food during chewing
• Mixing food with saliva and forming the bolus
• Initiation of swallowing, and speech

34
Tongue

• Intrinsic muscles change the shape of the tongue
• Extrinsic muscles alter the tongues position
• Lingual frenulum secures the tongue to the floor
  of the mouth

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Tongue

• Superior surface bears three types of papillae
• Filiform give the tongue roughness and provide
  friction
• Fungiform scattered widely over the tongue and
  give it a reddish hue
• Circumvallate V-shaped row in back of tongue

36
Tongue

• Sulcus terminalis groove that separates the
  tongue into two areas
• Anterior 2/3 residing in the oral cavity
• Posterior third residing in the oropharynx

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Tongue
Figure 23.8
38
Salivary Glands

• Produce and secrete saliva that
• Cleanses the mouth
• Moistens and dissolves food chemicals
• Aids in bolus formation
• Contains enzymes that break down starch

39
Salivary Glands

• Three pairs of extrinsic glands parotid,
  submandibular, and sublingual
• Intrinsic salivary glands (buccal glands)
  scattered throughout the oral mucosa

40
Salivary Glands

• Parotid lies anterior to the ear between the
  masseter muscle and skin
• Parotid duct opens into the vestibule next to
  second upper molar
• Submandibular lies along the medial aspect of
  the mandibular body
• Its ducts open at the base of the lingual
  frenulum
• Sublingual lies anterior to the submandibular
  gland under the tongue
• It opens via 10-12 ducts into the floor of the
  mouth

41
Salivary Glands
Figure 23.9a
42
Saliva Source and Composition

• Secreted from serous and mucous cells of salivary
  glands
• 97-99.5 water, hypo-osmotic, slightly acidic
  solution containing
• Electrolytes Na, K, Cl, PO42, HCO3
• Digestive enzyme salivary amylase
• Proteins mucin, lysozyme, defensins, and IgA
• Metabolic wastes urea and uric acid

43
Control of Salivation

• Intrinsic glands keep the mouth moist
• Extrinsic salivary glands secrete serous,
  enzyme-rich saliva in response to
• Ingested food which stimulates chemoreceptors and
  pressoreceptors
• The thought of food
• Strong sympathetic stimulation inhibits
  salivation and results in dry mouth

44
Teeth

• Primary and permanent dentitions have formed by
  age 21
• Primary 20 deciduous teeth that erupt at
  intervals between 6 and 24 months
• Permanent enlarge and develop causing the root
  of deciduous teeth to be resorbed and fall out
  between the ages of 6 and 12 years
• All but the third molars have erupted by the end
  of adolescence
• Usually 32 permanent teeth

45
Deciduous Teeth
Figure 23.10.1
46
Permanent Teeth
Figure 23.10.2
47
Classification of Teeth

• Teeth are classified according to their shape and
  function
• Incisors chisel-shaped teeth for cutting or
  nipping
• Canines fanglike teeth that tear or pierce
• Premolars (bicuspids) and molars have broad
  crowns with rounded tips best suited for
  grinding or crushing
• During chewing, upper and lower molars lock
  together generating crushing force

48
Dental Formula Permanent Teeth

• A shorthand way of indicating the number and
  relative position of teeth
• Written as ratio of upper to lower teeth for the
  mouth
• Primary 2I (incisors), 1C (canine), 2M (molars)
• Permanent 2I, 1C, 2PM (premolars), 3M

49
Tooth Structure

• Two main regions crown and the root
• Crown exposed part of the tooth above the
  gingiva
• Enamel acellular, brittle material composed of
  calcium salts and hydroxyapatite crystals the
  hardest substance in the body
• Encapsules the crown of the tooth
• Root portion of the tooth embedded in the
  jawbone

50
Tooth Structure

• Neck constriction where the crown and root come
  together
• Cementum calcified connective tissue
• Covers the root
• Attaches it to the periodontal ligament

51
Tooth Structure

• Periodontal ligament
• Anchors the tooth in the alveolus of the jaw
• Forms the fibrous joint called a gomaphosis
• Gingival sulcus depression where the gingiva
  borders the tooth

52
Tooth Structure

• Dentin bonelike material deep to the enamel cap
  that forms the bulk of the tooth
• Pulp cavity cavity surrounded by dentin that
  contains pulp
• Pulp connective tissue, blood vessels, and
  nerves

53
Tooth Structure

• Root canal portion of the pulp cavity that
  extends into the root
• Apical foramen proximal opening to the root
  canal
• Odontoblasts secrete and maintain dentin
  throughout life

54
Tooth Structure
Figure 23.11
55
Tooth and Gum Disease

• Dental caries gradual demineralization of
  enamel and dentin by bacterial action
• Dental plaque, a film of sugar, bacteria, and
  mouth debris, adheres to teeth
• Acid produced by the bacteria in the plaque
  dissolves calcium salts
• Without these salts, organic matter is digested
  by proteolytic enzymes
• Daily flossing and brushing help prevent caries
  by removing forming plaque

56
Tooth and Gum Disease Periodontitis

• Gingivitis as plaque accumulates, it calcifies
  and forms calculus, or tartar
• Accumulation of calculus
• Disrupts the seal between the gingivae and the
  teeth
• Puts the gums at risk for infection
• Periodontitis serious gum disease resulting
  from an immune response
• Immune system attacks intruders as well as body
  tissues, carving pockets around the teeth and
  dissolving bone

57
Pharynx

• From the mouth, the oro- and laryngopharynx allow
  passage of
• Food and fluids to the esophagus
• Air to the trachea
• Lined with stratified squamous epithelium and
  mucus glands
• Has two skeletal muscle layers
• Inner longitudinal
• Outer pharyngeal constrictors

58
Esophagus

• Muscular tube going from the laryngopharynx to
  the stomach
• Travels through the mediastinum and pierces the
  diaphragm
• Joins the stomach at the cardiac orifice

59
Esophagus
Figure 23.12
60
Esophageal Characteristics

• Esophageal mucosa nonkeratinized stratified
  squamous epithelium
• The empty esophagus is folded longitudinally and
  flattens when food is present
• Glands secrete mucus as a bolus moves through the
  esophagus
• Muscularis changes from skeletal (superiorly) to
  smooth muscle (inferiorly)

61
Digestive Processes in the Mouth

• Food is ingested
• Mechanical digestion begins (chewing)
• Propulsion is initiated by swallowing
• Salivary amylase begins chemical breakdown of
  starch
• The pharynx and esophagus serve as conduits to
  pass food from the mouth to the stomach

62
Deglutition (Swallowing)

• Coordinated activity of the tongue, soft palate,
  pharynx, esophagus, and 22 separate muscle groups
• Buccal phase bolus is forced into the
  oropharynx

63
Deglutition (Swallowing)

• Pharyngeal-esophageal phase controlled by the
  medulla and lower pons
• All routes except into the digestive tract are
  sealed off
• Peristalsis moves food through the pharynx to the
  esophagus

64
Bolus of food
Tongue
Uvula
Pharynx
Bolus
Epiglottis
Epiglottis
Glottis
Esophagus
Trachea
Bolus
(a)
(b)
(c)
Upper esophageal sphincter contracted
Upper esophageal sphincter relaxed
Upper esophageal sphincter contracted
Relaxed muscles
Relaxed muscles
Circular muscles contract, constricting passageway
and pushing bolus down
Gastroesophageal sphincter open
Bolus of food
Longitudinal muscles contract, shortening passagew
ay ahead of bolus
Stomach
Gastroesophageal sphincter closed
(d)
(e)
Figure 23.13
65
Deglutition (Swallowing)
Bolus of food
Tongue
Pharynx
Epiglottis
Glottis
Trachea
(a)
Upper esophagea sphincter contracted
Figure 23.13
66
Deglutition (Swallowing)
Uvula
Bolus
Epiglottis
Esophagus
(b)
Upper esophageal sphincter relaxed
Figure 23.13
67
Deglutition (Swallowing)
Bolus
(c)
Upper esophageal sphincter contracted
Figure 23.13
68
Deglutition (Swallowing)
Relaxed muscles
Circular muscles contract, constricting passageway
and pushing bolus down
Bolus of food
Longitudinal muscles contract, shortening passagew
ay ahead of bolus
Stomach
Gastroesophageal sphincter closed
(d)
Figure 23.13
69
Deglutition (Swallowing)
Relaxed muscles
Gastroesophageal sphincter open
(e)
Figure 23.13
70
Bolus of food
Tongue
Uvula
Pharynx
Bolus
Epiglottis
Epiglottis
Glottis
Esophagus
Trachea
Bolus
(a)
(b)
(c)
Upper esophageal sphincter contracted
Upper esophageal sphincter relaxed
Upper esophageal sphincter contracted
Relaxed muscles
Relaxed muscles
Circular muscles contract, constricting passageway
and pushing bolus down
Bolus of food
Longitudinal muscles contract, shortening passagew
ay ahead of bolus
Stomach
Gastroesophageal sphincter closed
(d)
(e)
Figure 23.13
71
Stomach

• Chemical breakdown of proteins begins and food is
  converted to chyme
• Cardiac region surrounds the cardiac orifice
• Fundus dome-shaped region beneath the diaphragm
• Body midportion of the stomach
• Pyloric region made up of the antrum and canal
  which terminates at the pylorus
• The pylorus is continuous with the duodenum
  through the pyloric sphincter

72
Stomach

• Greater curvature entire extent of the convex
  lateral surface
• Lesser curvature concave medial surface
• Lesser omentum runs from the liver to the
  lesser curvature
• Greater omentum drapes inferiorly from the
  greater curvature to the small intestine

73
Stomach

• Nerve supply sympathetic and parasympathetic
  fibers of the autonomic nervous system
• Blood supply celiac trunk, and corresponding
  veins (part of the hepatic portal system)

74
Figure 23.14a
75
Microscopic Anatomy of the Stomach

• Muscularis has an additional oblique layer
  that
• Allows the stomach to churn, mix, and pummel food
  physically
• Breaks down food into smaller fragments

76
Microscopic Anatomy of the Stomach

• Epithelial lining is composed of
• Goblet cells that produce a coat of alkaline
  mucus
• The mucous surface layer traps a bicarbonate-rich
  fluid beneath it
• Gastric pits contain gastric glands that secrete
  gastric juice, mucus, and gastrin

77
Microscopic Anatomy of the Stomach
Figure 23.15a
78
Microscopic Anatomy of the Stomach
Figure 23.15b
79
Microscopic Anatomy of the Stomach
Figure 23.15c
80
Glands of the Stomach Fundus and Body

• Gastric glands of the fundus and body have a
  variety of secretory cells
• Mucous neck cells secrete acid mucus
• Parietal cells secrete HCl and intrinsic factor

81
Glands of the Stomach Fundus and Body

• Chief cells produce pepsinogen
• Pepsinogen is activated to pepsin by
• HCl in the stomach
• Pepsin itself via a positive feedback mechanism
• Enteroendocrine cells secrete gastrin,
  histamine, endorphins, serotonin, cholecystokinin
  (CCK), and somatostatin into the lamina propria

82
Stomach Lining

• The stomach is exposed to the harshest conditions
  in the digestive tract
• To keep from digesting itself, the stomach has a
  mucosal barrier with
• A thick coat of bicarbonate-rich mucus on the
  stomach wall
• Epithelial cells that are joined by tight
  junctions
• Gastric glands that have cells impermeable to HCl
• Damaged epithelial cells are quickly replaced

83
Digestion in the Stomach

• The stomach
• Holds ingested food
• Degrades this food both physically and chemically
• Delivers chyme to the small intestine
• Enzymatically digests proteins with pepsin
• Secretes intrinsic factor required for absorption
  of vitamin B12

84
Regulation of Gastric Secretion

• Neural and hormonal mechanisms regulate the
  release of gastric juice
• Stimulatory and inhibitory events occur in three
  phases
• Cephalic (reflex) phase prior to food entry
• Gastric phase once food enters the stomach
• Intestinal phase as partially digested food
  enters the duodenum

85
Cephalic Phase

• Excitatory events include
• Sight or thought of food
• Stimulation of taste or smell receptors
• Inhibitory events include
• Loss of appetite or depression
• Decrease in stimulation of the parasympathetic
  division

86
Gastric Phase

• Excitatory events include
• Stomach distension
• Activation of stretch receptors (neural
  activation)
• Activation of chemoreceptors by peptides,
  caffeine, and rising pH
• Release of gastrin to the blood

87
Gastric Phase

• Inhibitory events include
• A pH lower than 2
• Emotional upset that overrides the
  parasympathetic division

88
Intestinal Phase

• Excitatory phase low pH partially digested
  food enters the duodenum and encourages gastric
  gland activity
• Inhibitory phase distension of duodenum,
  presence of fatty, acidic, or hypertonic chyme,
  and/or irritants in the duodenum
• Initiates inhibition of local reflexes and vagal
  nuclei
• Closes the pyloric sphincter
• Releases enterogastrones that inhibit gastric
  secretion

89
Release of Gastric Juice Stimulatory Events
Figure 23.16.1
90
Release of Gastric Juice Inhibitory Events
Figure 23.16.2
91
Regulation and Mechanism of HCl Secretion

• HCl secretion is stimulated by ACh, histamine,
  and gastrin through second-messenger systems
• Release of hydrochloric acid
• Is low if only one ligand binds to parietal cells
• Is high if all three ligands bind to parietal
  cells
• Antihistamines block H2 receptors and decrease
  HCl release

92
Response of the Stomach to Filling

• Stomach pressure remains constant until about 1L
  of food is ingested
• Relative unchanging pressure results from
  reflex-mediated relaxation and plasticity

93
Response of the Stomach to Filling

• Reflex-mediated events include
• Receptive relaxation as food travels in the
  esophagus, stomach muscles relax
• Adaptive relaxation the stomach dilates in
  response to gastric filling
• Plasticity intrinsic ability of smooth muscle
  to exhibit the stress-relaxation response

94
Gastric Contractile Activity

• Peristaltic waves move toward the pylorus at the
  rate of 3 per minute
• This basic electrical rhythm (BER) is initiated
  by pacemaker cells (cells of Cajal)

95
Gastric Contractile Activity

• Most vigorous peristalsis and mixing occurs near
  the pylorus
• Chyme is either
• Delivered in small amounts to the duodenum or
• Forced backward into the stomach for further
  mixing

96
Gastric Contractile Activity
Figure 23.18
97
Regulation of Gastric Emptying

• Gastric emptying is regulated by
• The neural enterogastric reflex
• Hormonal (enterogastrone) mechanisms
• These mechanisms inhibit gastric secretion and
  duodenal filling

98
Regulation of Gastric Emptying

• Carbohydrate-rich chyme quickly moves through the
  duodenum
• Fat-laden chyme is digested more slowly causing
  food to remain in the stomach longer

99
Small Intestine Gross Anatomy

• Runs from pyloric sphincter to the ileocecal
  valve
• Has three subdivisions duodenum, jejunum, and
  ileum

100
Small Intestine Gross Anatomy

• The bile duct and main pancreatic duct
• Join the duodenum at the hepatopancreatic ampulla
  
• Are controlled by the sphincter of Oddi
• The jejunum extends from the duodenum to the
  ileum
• The ileum joins the large intestine at the
  ileocecal valve

101
Small Intestine Microscopic Anatomy

• Structural modifications of the small intestine
  wall increase surface area
• Plicae circulares deep circular folds of the
  mucosa and submucosa
• Villi fingerlike extensions of the mucosa
• Microvilli tiny projections of absorptive
  mucosal cells plasma membranes

102
Duodenum and Related Organs
Figure 23.20
103
Small Intestine Microscopic Anatomy
Figure 23.21
104
Small Intestine Histology of the Wall

• The epithelium of the mucosa is made up of
• Absorptive cells and goblet cells
• Enteroendocrine cells
• Interspersed T cells called intraepithelial
  lymphocytes (IELs)
• IELs immediately release cytokines upon
  encountering Ag

105
Small Intestine Histology of the Wall

• Cells of intestinal crypts secrete intestinal
  juice
• Peyers patches are found in the submucosa
• Brunners glands in the duodenum secrete alkaline
  mucus

106
Intestinal Juice

• Secreted by intestinal glands in response to
  distension or irritation of the mucosa
• Slightly alkaline and isotonic with blood plasma
• Largely water, enzyme-poor, but contains mucus

107
Liver

• The largest gland in the body
• Superficially has four lobes right, left,
  caudate, and quadrate
• The falciform ligament
• Separates the right and left lobes anteriorly
• Suspends the liver from the diaphragm and
  anterior abdominal wall

108
Liver

• The ligamentum teres
• Is a remnant of the fetal umbilical vein
• Runs along the free edge of the falciform ligament

109
Liver Associated Structures

• The lesser omentum anchors the liver to the
  stomach
• The hepatic blood vessels enter the liver at the
  porta hepatis
• The gallbladder rests in a recess on the inferior
  surface of the right lobe

110
Liver Associated Structures

• Bile leaves the liver via
• Bile ducts, which fuse into the common hepatic
  duct
• The common hepatic duct, which fuses with the
  cystic duct
• These two ducts form the bile duct

111
Gallbladder and Associated Ducts
Figure 23.20
112
Liver Microscopic Anatomy

• Hexagonal-shaped liver lobules are the structural
  and functional units of the liver
• Composed of hepatocyte (liver cell) plates
  radiating outward from a central vein
• Portal triads are found at each of the six
  corners of each liver lobule

Figure 23.24c
113
Liver Microscopic Anatomy

• Portal triads consist of a bile duct and
• Hepatic artery supplies oxygen-rich blood to
  the liver
• Hepatic portal vein carries venous blood with
  nutrients from digestive viscera

Figure 23.24d
114
Liver Microscopic Anatomy

• Liver sinusoids enlarged, leaky capillaries
  located between hepatic plates
• Kupffer cells hepatic macrophages found in
  liver sinusoids

115
Liver Microscopic Anatomy

• Hepatocytes functions include
• Production of bile
• Processing bloodborne nutrients
• Storage of fat-soluble vitamins
• Detoxification
• Secreted bile flows between hepatocytes toward
  the bile ducts in the portal triads

116
Microscopic Anatomy of the Liver
Figure 23.24c, d
117
Composition of Bile

• A yellow-green, alkaline solution containing bile
  salts, bile pigments, cholesterol, neutral fats,
  phospholipids, and electrolytes
• Bile salts are cholesterol derivatives that
• Emulsify fat
• Facilitate fat and cholesterol absorption
• Help solubilize cholesterol
• Enterohepatic circulation recycles bile salts
• The chief bile pigment is bilirubin, a waste
  product of heme

118
The Gallbladder

• Thin-walled, green muscular sac on the ventral
  surface of the liver
• Stores and concentrates bile by absorbing its
  water and ions
• Releases bile via the cystic duct, which flows
  into the bile duct

119
Regulation of Bile Release

• Acidic, fatty chyme causes the duodenum to
  release
• Cholecystokinin (CCK) and secretin into the
  bloodstream
• Bile salts and secretin transported in blood
  stimulate the liver to produce bile
• Vagal stimulation causes weak contractions of the
  gallbladder

120
Regulation of Bile Release

• Cholecystokinin causes
• The gallbladder to contract
• The hepatopancreatic sphincter to relax
• As a result, bile enters the duodenum

121
Regulation of Bile Release
Vagal stimulation causes weak contractions of
gallbladder
4
Bile salts and secretin transported
via bloodstream stimulate liver to produce
bile more rapidly
3
Cholecystokinin (via bloodstream) causes
gallbladder to contract and hepatopancreatic sphin
cter to relax bile enters duodenum
5
Acidic, fatty chyme entering duodenum
causes release of cholecystokinin and secretin
from duodenal wall enteroendocrine cells
1
Cholecystokinin and secretin enter the
bloodstream
2
Bile salts reabsorbed into blood
6
Figure 23.25
122
Pancreas

• Location
• Lies deep to the greater curvature of the stomach
• The head is encircled by the duodenum and the
  tail abuts the spleen

123
Pancreas

• Exocrine function
• Secretes pancreatic juice which breaks down all
  categories of foodstuff
• Acini (clusters of secretory cells) contain
  zymogen granules with digestive enzymes
• The pancreas also has an endocrine function
  release of insulin and glucagon

124
Acinus of the Pancreas
Figure 23.26a
125
Pancreatic Activation
Figure 23.27
126
Composition and Function of Pancreatic Juice

• Water solution of enzymes and electrolytes
  (primarily HCO3)
• Neutralizes acid chyme
• Provides optimal environment for pancreatic
  enzymes
• Enzymes are released in inactive form and
  activated in the duodenum

127
Composition and Function of Pancreatic Juice

• Examples include
• Trypsinogen is activated to trypsin
• Procarboxypeptidase is activated to
  carboxypeptidase
• Active enzymes secreted
• Amylase, lipases, and nucleases
• These enzymes require ions or bile for optimal
  activity

128
Regulation of Pancreatic Secretion

• Secretin and CCK are released when fatty or
  acidic chyme enters the duodenum
• CCK and secretin enter the bloodstream
• Upon reaching the pancreas
• CCK induces the secretion of enzyme-rich
  pancreatic juice
• Secretin causes secretion of bicarbonate-rich
  pancreatic juice
• Vagal stimulation also causes release of
  pancreatic juice

129
Regulation of Pancreatic Secretion
During cephalic and gastric phases, stimulation
by vagal nerve fibers causes release of
pancreatic juice and weak contractions of the
gallbladder.
1
Acidic chyme entering duodenum causes
the enteroendocrine cells of the duodenal wall to
release secretin, whereas fatty, protein-rich
chyme induces release of cholecystokinin.
Cholecystokinin and secretin
enter bloodstream.
2
3
Upon reaching the pancreas,
cholecystokinin induces the secretion
of enzyme-rich pancreatic juice secretin causes
copious secretion of bicarbonate-rich pancreatic
juice.
Figure 23.28
130
Digestion in the Small Intestine

• As chyme enters the duodenum
• Carbohydrates and proteins are only partially
  digested
• No fat digestion has taken place

131
Digestion in the Small Intestine

• Digestion continues in the small intestine
• Chyme is released slowly into the duodenum
• Because it is hypertonic and has low pH, mixing
  is required for proper digestion
• Required substances needed are supplied by the
  liver
• Virtually all nutrient absorption takes place in
  the small intestine

132
Motility in the Small Intestine

• The most common motion of the small intestine is
  segmentation
• It is initiated by intrinsic pacemaker cells
  (Cajal cells)
• Moves contents steadily toward the ileocecal valve

133
Motility in the Small Intestine

• After nutrients have been absorbed
• Peristalsis begins with each wave starting distal
  to the previous
• Meal remnants, bacteria, mucosal cells, and
  debris are moved into the large intestine

134
Control of Motility

• Local enteric neurons of the GI tract coordinate
  intestinal motility
• Cholinergic neurons cause
• Contraction and shortening of the circular muscle
  layer
• Shortening of longitudinal muscle
• Distension of the intestine

135
Control of Motility

• Other impulses relax the circular muscle
• The gastroileal reflex and gastrin
• Relax the ileocecal sphincter
• Allow chyme to pass into the large intestine

136
Large Intestine

• Has three unique features
• Teniae coli three bands of longitudinal smooth
  muscle in its muscularis
• Haustra pocketlike sacs caused by the tone of
  the teniae coli
• Epiploic appendages fat-filled pouches of
  visceral peritoneum

137
Large Intestine

• Is subdivided into the cecum, appendix, colon,
  rectum, and anal canal
• The saclike cecum
• Lies below the ileocecal valve in the right iliac
  fossa
• Contains a wormlike vermiform appendix

138
Large Intestine
Figure 23.29a
139
Colon

• Has distinct regions ascending colon, hepatic
  flexure, transverse colon, splenic flexure,
  descending colon, and sigmoid colon
• The transverse and sigmoid portions are anchored
  via mesenteries called mesocolons
• The sigmoid colon joins the rectum
• The anal canal, the last segment of the large
  intestine, opens to the exterior at the anus

140
Valves and Sphincters of the Rectum and Anus

• Three valves of the rectum stop feces from being
  passed with gas
• The anus has two sphincters
• Internal anal sphincter composed of smooth muscle
• External anal sphincter composed of skeletal
  muscle
• These sphincters are closed except during
  defecation

141
Mesenteries of Digestive Organs
Figure 23.30b
142
Mesenteries of Digestive Organs
Figure 23.30c
143
Mesenteries of Digestive Organs
Figure 23.30d
144
Large Intestine Microscopic Anatomy

• Colon mucosa is simple columnar epithelium except
  in the anal canal
• Has numerous deep crypts lined with goblet cells

145
Large Intestine Microscopic Anatomy

• Anal canal mucosa is stratified squamous
  epithelium
• Anal sinuses exude mucus and compress feces
• Superficial venous plexuses are associated with
  the anal canal
• Inflammation of these veins results in itchy
  varicosities called hemorrhoids

146
Structure of the Anal Canal
Figure 23.29b
147
Bacterial Flora

• The bacterial flora of the large intestine
  consist of
• Bacteria surviving the small intestine that enter
  the cecum and
• Those entering via the anus
• These bacteria
• Colonize the colon
• Ferment indigestible carbohydrates
• Release irritating acids and gases (flatus)
• Synthesize B complex vitamins and vitamin K

148
Functions of the Large Intestine

• Other than digestion of enteric bacteria, no
  further digestion takes place
• Vitamins, water, and electrolytes are reclaimed
• Its major function is propulsion of fecal
  material toward the anus
• Though essential for comfort, the colon is not
  essential for life

149
Motility of the Large Intestine

• Haustral contractions
• Slow segmenting movements that move the contents
  of the colon
• Haustra sequentially contract as they are
  stimulated by distension
• Presence of food in the stomach
• Activates the gastrocolic reflex
• Initiates peristalsis that forces contents toward
  the rectum

150
Defecation

• Distension of rectal walls caused by feces
• Stimulates contraction of the rectal walls
• Relaxes the internal anal sphincter
• Voluntary signals stimulate relaxation of the
  external anal sphincter and defecation occurs

151
Defecation
Figure 23.32
152
Chemical Digestion Carbohydrates

• Absorption via cotransport with Na, and
  facilitated diffusion
• Enter the capillary bed in the villi
• Transported to the liver via the hepatic portal
  vein
• Enzymes used salivary amylase, pancreatic
  amylase, and brush border enzymes

153
Chemical Digestion Proteins

• Absorption similar to carbohydrates
• Enzymes used pepsin in the stomach
• Enzymes acting in the small intestine
• Pancreatic enzymes trypsin, chymotrypsin, and
  carboxypeptidase
• Brush border enzymes aminopeptidases,
  carboxypeptidases, and dipeptidases

154
Figure 23.34
155
Chemical Digestion Fats

• Absorption Diffusion into intestinal cells where
  they
• Combine with proteins and extrude chylomicrons
• Enter lacteals and are transported to systemic
  circulation via lymph

156
Chemical Digestion Fats

• Glycerol and short chain fatty acids are
• Absorbed into the capillary blood in villi
• Transported via the hepatic portal vein
• Enzymes/chemicals used bile salts and pancreatic
  lipase

157
Chemical Digestion Fats
Figure 23.35
158
Fatty Acid Absorption

• Fatty acids and monoglycerides enter intestinal
  cells via diffusion
• They are combined with proteins within the cells
• Resulting chylomicrons are extruded
• They enter lacteals and are transported to the
  circulation via lymph

159
Fatty Acid Absorption
Fatty acids and monoglycerides associated
with micelles in lumen of intestine
Lumen of intestine
Fatty acids and monoglycerides resulting from
fat digestion leave micelles and enter epithelial
cell by diffusion.
1
Absorptive epithelial cell cytoplasm
Fatty acids are used to synthesize
triglycerides in smooth endo- plasmic reticulum.
2
ER
Golgi apparatus
Fatty globules are combined with proteins to
form chylomicrons (within Golgi apparatus).
3
Vesicles containing chylomicrons migrate to the
basal membrane, are extruded from the
epithelial cell, and enter a lacteal (lymphatic
capillary).
4
Lymph in the lacteal transports chylomicrons
away from intestine.
5
Chylomicron
Lacteal
Figure 23.36
160
Chemical Digestion Nucleic Acids

• Absorption active transport via membrane
  carriers
• Absorbed in villi and transported to liver via
  hepatic portal vein
• Enzymes used pancreatic ribonucleases and
  deoxyribonuclease in the small intestines

161
Electrolyte Absorption

• Most ions are actively absorbed along the length
  of small intestine
• Na is coupled with absorption of glucose and
  amino acids
• Ionic iron is transported into mucosal cells
  where it binds to ferritin
• Anions passively follow the electrical potential
  established by Na

162
Electrolyte Absorption

• K diffuses across the intestinal mucosa in
  response to osmotic gradients
• Ca2 absorption
• Is related to blood levels of ionic calcium
• Is regulated by vitamin D and parathyroid hormone
  (PTH)

163
Water Absorption

• 95 of water is absorbed in the small intestines
  by osmosis
• Water moves in both directions across intestinal
  mucosa
• Net osmosis occurs whenever a concentration
  gradient is established by active transport of
  solutes into the mucosal cells
• Water uptake is coupled with solute uptake, and
  as water moves into mucosal cells, substances
  follow along their concentration gradients

164
Malabsorption of Nutrients

• Results from anything that interferes with
  delivery of bile or pancreatic juice
• Factors that damage the intestinal mucosa (e.g.,
  bacterial infection)
• Gluten enteropathy (adult celiac disease)
  gluten damages the intestinal villi and reduces
  the length of microvilli
• Treated by eliminating gluten from the diet (all
  grains but rice, corn, quinoa, buckwheat)

165
Cancer

• Stomach and colon cancers rarely have early signs
  or symptoms
• Metastasized colon cancers frequently cause
  secondary liver cancer
• Prevention is by regular dental and medical
  examinations

166
Cancer

• Colon cancer is the 2nd largest cause of cancer
  deaths in males (lung cancer is 1st)
• Forms from benign mucosal tumors called polyps
  whose formation increases with age
• Regular colon examination should be done for all
  those over 50